Wireless sensor reader

ABSTRACT

A wireless sensor reader is provided to interface with a wireless sensor. The wireless sensor reader transmits an excitation pulse to cause the wireless sensor to generate a ring signal. The wireless sensor reader receives and amplifies the ring signal and sends the signal to a phase-locked loop. A voltage-controlled oscillator in the phase-locked loop locks onto the ring signal frequency and generates a count signal at a frequency related to the ring signal frequency. The voltage-controlled oscillator is placed into a hold mode where the control voltage is maintained constant to allow the count signal frequency to be determined.

RELATED APPLICATIONS

This non-provisional application is a continuation of U.S. patentapplication Ser. No. 12/419,326 filed on Apr. 7, 2009, which is acontinuation-in-part of U.S. patent application Ser. No. 12/075,858filed on Mar. 14, 2008, which claims priority to U.S. ProvisionalApplication No. 60/918,164 filed on Mar. 15, 2007.

TECHNICAL FIELD

This invention relates generally to reading passive wireless sensors,and more particularly to a reader circuitry for exciting and sensingdata from passive wireless sensors.

BACKGROUND

Passive wireless sensor systems that employ resonant circuit technologyare known. These systems utilize a passive wireless sensor in remotecommunication with excitation and reader circuitry. Often the wirelesssensor is implanted at a specific location, such as within the humanbody, to detect and report a sensed parameter. The sensed parametervaries the resonant circuit frequency of the wireless sensor. The readerdevice samples the resonant frequency of the wireless sensor todetermine the sensed parameter.

U.S. Pat. No. 4,127,110 by Bullara discloses a sensor for measuringbrain fluid pressure measurement. U.S. Pat. No. 4,206,762 by Cosmandiscloses a similar sensor for measuring internal pressure.Specifically, the Cosman patent describes the use of a grid dip systemfor wirelessly measuring the resonant frequency of the sensor. Inaddition, the Cosman patent discloses the possibility of a batterypowered portable reader device.

Several methods of reading passive wireless sensors have also beendescribed in prior patents. For example, the Cosman patent discloses anexternal oscillator circuit that uses the implanted sensor for tuning,and a grid dip measurement system for measurement of sensor resonantfrequency. U.S. Pat. No. 6,015,386 by Kensey, et al., discloses a readerthat excites the passive sensor by transmitting frequency sweeps anduses a phase detector on the transmit signal to detect the time duringthe sweep where the transmitted frequency matches the resonancefrequency of the sensor. U.S. Pat. No. 6,206,835 by Spillman, et al.,discloses a medical implant application for reader technology disclosedin U.S. Pat. No. 5,581,248 by Spillman, et al. This reader technologydetects a frequency dependent variable impedance loading effect on thereader by the sensor's detected parameter. U.S. Pat. No. 7,432,723 byEllis, et al., discloses a reader with energizing loops each tuned toand transmitting different frequencies spaced to ensure that thebandwidth of the sensor allows resonant excitation of the sensor. Ellisuses a ring-down response from the appropriate energizing loop todetermine the sensor resonant frequency.

Some readers utilize phased-locked-loop (“PLL”) circuitry to lock ontothe sensor's resonant frequency. U.S. Pat. No. 7,245,117 by Joy, et al.discloses an active PLL circuit and signal processing circuit thatadjusts a transmitting PLL frequency until the received signal phase andthe transmitting PLL signal phase match. When this match occurs, thetransmitting PLL frequency is equal to the sensor resonant frequency.

PLL circuits may incorporate sample and hold (S/H) functions to samplethe input frequency and hold the PLL at a given frequency. PLLs with S/Hmay be used in a variety of applications. For example, U.S. Pat. No.4,531,526 by Genest discloses a reader that uses a PLL circuit with aS/H circuit to adjust the transmitted frequency of the reader to matchthe resonant frequency received from the sensor. This is done tomaximize sensor response to the next transmission and measures the decayrate of the sensor resonance amplitude to extract the sensor value. U.S.Pat. No. 4,644,420 by Buchan describes a PLL with a S/H used to sample atape data stream and maintain an appropriate sampling frequency forevaluation of digital data pulses on the tape. U.S. Pat. No. 5,006,819by Buchan, et al., provides additional enhancements to this concept.U.S. Pat. No. 5,920,233 by Denny describes a high-speed samplingtechnique using a S/H circuit with a PLL to reduce the charge pump noisefrom the phase-frequency detector to enhance the low jitter performanceof a frequency synthesizing circuit. U.S. Pat. No. 4,511,858 by Charvit,et al., discloses a PLL with a S/H circuit to pre-position the controlvoltage of a voltage controlled oscillator when the PLL lock frequencyis being changed. This is done to enhance the response speed of the PLLwhen changing the desired synthesized frequency. U.S. Pat. No. 6,570,457by Fischer and U.S. Pat. No. 6,680,654 by Fischer, et al., disclose aPLL with S/H circuitry to enhance PLL frequency stepping as well asoffset correction feature. U.S. Pat. No. 3,872,455 by Fuller, et al.discloses a PLL having a digital S/H to freeze the frequency display andpreload the frequency counter when a PLL phase lock is detected.

Current designs for passive sensor readers, such as those disclosedabove, suffer from a number of deficiencies. Swept frequency sensorreaders similar to those described in the Cosman, Kensey, Ellis andSpillman patents require relatively wide bandwidth allowance by thegovernment body regulating radio transmissions. This limits other usesof the spectrum and makes interference a potential issue. Readers thattrack the resonant frequency of a passive resonant sensor with avariable frequency transmitter, such as Genest, Ellis, and Joy alsosuffer from similar problems. Moreover, the amount of electrical powerneeded for transmissions, signal processing, sampling, and tracking theresonant frequency of a sensor using digitally controlled frequencytracking or swept frequency systems is significant and limits theability to use battery power in a reader, as well as limiting thelongevity of batteries in a battery powered reader. Accordingly, animproved passive sensor reader is needed in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference to the detailed description is taken in connection with thefollowing illustrations:

FIG. 1 illustrates a block diagram of a passive wireless sensor system;

FIG. 2 illustrates a block diagram of the reader circuitry;

FIG. 3 illustrates a block diagram of the timing and control portion ofthe reader circuitry;

FIG. 4 illustrates a block diagram of the transmit portion of the readercircuitry;

FIG. 5 illustrates a block diagram of the receive portion of the readercircuitry;

FIG. 6 illustrates a block diagram of the phase locked loop portion ofthe reader circuitry; and

FIG. 7 illustrates a block diagram of the frequency counter portion ofthe reader circuitry.

SUMMARY

A reader device is provided to interface with a wireless sensor. Thereader emits a short pulse of energy or a short burst of radio frequencyenergy to cause the wireless sensor to ring. Immediately after thetransmission, the reader receives and amplifies the sensor signal, thensends the signal to a phase-locked loop (“PLL”) that locks to the sensorring frequency. Once the PLL has locked to the ring frequency, the PLL'svoltage controlled oscillator (“VCO”) is placed in a hold mode tomaintain the VCO frequency at the locked frequency. The VCO frequency iscounted to determine the sensor resonant frequency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A passive wireless sensor system including a reader 10 in remotecommunication with a sensor 12 is provided. The reader is capable ofexciting the sensor 12 by transmitting a signal, such as a radiofrequency (“RF”) pulse, at or near the resonant frequency of the sensor12. (See FIG. 1.) The sensor 12 may emit a ring frequency for a shortperiod of time in response to the excitation pulse from the reader 10.

The sensor 12 may be a passive device, capable of emitting a ring signalin response to an excitation signal at or near the resonant frequency ofthe sensor 12. The sensor 12 may be configured to sense a specificparameter. For example, the sensor 12 may include a fixed inductor and acapacitor that varies based on the sensed parameter. The varyingcapacitance alters the resonant and ring frequencies of the sensor 12.It should be appreciated, however, that the sensor 12 may be anywireless sensor known in the art capable of remote communication withthe reader 10. Further, while the sensor 12 is described as an RFresonant sensor, it will be appreciated that the sensor 12 may be aacoustically resonant sensor, optically resonant sensor, or othersimilar sensor known in the art. The corresponding reader 10 may employcorresponding signals to activate the sensor 12. Further, the sensor 12may be an active sensor or a passive sensor.

The reader 10 may excite the sensor 12 by transmitting an excitationpulse 14 in the vicinity of the sensor 12. For example, the reader mayemit a radio frequency (“RF”) excitation pulse 14 at or near theresonant frequency of the sensor 12. The sensor 12 may emit a ringsignal 16 in response to the excitation pulse 14. The reader 10 maydetermine the frequency of the ring signal 16 in order to determine thesensed parameter value.

The reader 10 may also communicate with a data interface 17. The reader10 and data interface 17 may be connected directly or indirectly, or maycommunicate via a remote connection. The reader 10 may send information,such as data related to the sensor 12, to the data interface 17. Thereader 10 may further send information regarding the status of thereader 10 to the data interface 17. The data interface 17 may provideconfiguration information to the reader 10. For example, the datainterface 17 may provide information regarding schedules and intervalsfor sampling the sensor 12.

The data interface 17 may communicate with a remote data system 18 toexchange status and control signals, as well as provide sensor data. Theremote data system 18 may include a data gathering module 19 to receivedata from the data interface 17, a data logging module 20 to store thereceived data, and a data display 21 to display the sensor data.

The reader 10 includes circuitry to send the excitation pulse 14,receive the ring signal 16, and process the ring signal 16. (FIG. 2.)For example, the reader 10 includes a timing and control circuitry 22 toconfigure and activate the other circuits in the reader 10. The solidarrows between the timing and control circuitry 22 represent the controlinterfaces, such as digital or low-frequency signals. The timing andcontrol circuitry 22 further generates an RF signal (illustrated as thebroken line arrow) that is sent to a transmit circuitry 24. The transmitcircuitry 24 receives the RF signal and sends out the excitation pulse14 to excite the sensor 12. The timing and control circuitry 22 may onlyprovide the RF signal to the transmit circuitry 24 during the intervalswhen the excitation pulse is being transmitted to prevent leakage orcoupling.

The reader 10 further includes an antenna 26 connected to the transmitcircuitry 24 and a receive circuitry 28. The transmit circuitry 24utilizes the antenna 26 for transmitting the excitation pulse 14, whilethe receive circuitry 28 utilizes the antenna 26 for receiving the ringsignal 16. In an embodiment, the antenna 26 is connected to both thetransmit circuitry 24 and the receive circuitry 28 at all times insteadof being switched between transmit and receive. This shared antenna 26design requires special consideration to prevent damage to the receivecircuitry 28. Specifically, care must be taken not to overload the verysensitive amplifier stages of the receive circuitry 28. Additionally,the reader 10 requires a fast transition between the extreme overdrivecondition present while the transmit circuitry 24 is driving the antenna26 and the low voltage condition present at the antenna 26 during thereceive and amplify phases. For instance, the voltage at the antenna 26may exceed 200 volts peak-to-peak during transmission of the excitationpulse, and may be single-digit millivolts, decaying rapidly tomicro-volts, during reception immediately following the excitation pulse14. While the reader 10 is described as having a shared antenna 26, itwill be appreciated that the reader 10 may incorporate more than oneantenna to separately perform the functions of transmitting theexcitation pulse 14 and receiving the ring signal 16.

The reader 10 further includes a PLL 30 to receive and lock onto thering signal 16. The receive circuitry 28 may amplify and condition thering signal 16 before sending it to the PLL 30. The PLL 30 includes avoltage controlled oscillator (“VCO”) 32 that operates at a frequencyhigher than the ring signal 16 frequency. The VCO 32 interfaces with afrequency counter 34 which counts the VCO 32 frequency, and provides thecount to a external interface circuitry 36 for transfer to the datainterface 17. By operating the VCO 32 at a higher frequency than thering signal 16, the time required to count and record the VCO 32frequency may be significantly decreased.

Each component of the reader 10 is designed to operate efficiently andreduce power consumption. To that end, the reader 10 includes a reducedpower mode to conserve power. The timing and control circuitry 22controls the power status of each component by way of a wakeup timer 38connected to each component. (FIG. 3.) In reduced power mode, somecomponents may be completely powered down while other components mayoperate in a sleep mode where power remains to maintain configurationbut the circuit becomes static to minimize power consumption.

The timing and control circuitry 22 may place each component of thereader 10 in a sleep or powered-down mode after a specified period ofinactivity, such as a few milliseconds when the reader 10 is notsampling the sensor 12. However, it will be appreciated that thespecified period of time before entering into reduced power mode may beadjustable. For example, the timing and control circuitry 22 may includea configuration buffer 40 that receives timing instructions from theexternal interface circuitry 36. The instructions establish the timingperiod before entering into reduced power mode, and other timing periodsfor the wakeup timer 38.

The wakeup timer 38 may wake up each component of the reader 10 at theappropriate time to ensure that each component is in an operationalstate when needed. Specifically, the wakeup timer 38 may communicatewith a transmit timer 42, a receive timer 46, a PLL timer 48, and afrequency counter timer 50 to wake up and control the respectivecomponents of the reader 10. Once initiated, each of these timers maycontrol and power up the respective component. When configured, thewakeup timer 38 may delay for a specified interval, which may be zeroseconds, before sending an initiate signal 52 to start the other timers.As illustrated in FIG. 3, the initiate signal 52 is not shown as acontinuous line from the wakeup timer 38 to the respective timers inorder to prevent line crossings and minimize confusion.

Once initiated, the transmit timer 42 establishes proper sequence andperiod to the power control 54, damp control 56, Q control 58, and RFenable 60 signals to properly sequence the transmit circuitry 24 andtransmit frequency generator 44. The power control signal 54 controlsthe power status and sleep status of the transmit circuitry 24. The dampcontrol signal 56 controls the activation of a damping circuit in thetransmit circuitry 24 to quickly dissipate antenna 26 energy at the endof a transmission period. The Q control signal 58 controls a switchingcircuit in the transmit circuitry 24 to reduce the Q of the antenna 26during reception of the ring signal 16. The RF enable signal allows thetransmit frequency generator 44 to send an RF signal to the transmitcircuitry 24. In an embodiment, the transmit frequency generator 44 onlyprovides the RF signal to the transmit circuitry 24 during periods wherethe transmit circuitry 24 is transmitting an excitation pulse 14.

The receive timer 46 is configured to establish proper sequence andperiod to the power control signal 62 to properly sequence the receivecircuitry 28.

The PLL timer 48 establishes proper sequence and period to the powercontrol 64 and S/H mode 66 signals to properly sequence the PLL 30. Thepower control signal 64 controls the power status and sleep status ofthe PLL 30. The S/H mode signal 66 controls a sample and hold circuit inthe PLL 30, used to cause the PLL to lock onto the transmitted frequencythen onto the ring signal 16 frequency, then hold the VCO 32 frequencyat the locked frequency until counting is complete.

The frequency counter timer 50 establishes proper sequence and countinterval to the power control 68 and start/stop count 70 signals toproperly sequence the frequency counter 34. The power control signal 68controls the power status and sleep status of the frequency counter 34.The start/stop count signal 70 controls the time that the frequencycounter 34 begins and ends counting the VCO 32 frequency.

The transmit circuitry 24 is configured to transmit the excitation pulse14 to the sensor 12 by way of the antenna 26. (FIG. 4.) The excitationpulse 14 may be a fixed or rapidly varying frequency burst at or nearthe resonant frequency of the sensor 12. For example, the excitationpulse 14 may be a fixed frequency burst within several bandwidths of thesensor 12 resonant frequency. Alternatively, the excitation pulse 14 maybe a fixed or rapidly varying frequency burst or sweep of a very shortduration at or near a frequency harmonically related to the sensor 12resonant frequency. The excitation pulse 14 may also be an ultra-wideband pulse. This plurality of excitation pulse 14 approaches iseffective because the ring signal 16 is received when the excitationpulse 14 transmissions have ceased. Therefore, excitation pulse 14transmissions may be limited to frequency bands, amplitudes, andmodulation schemes acceptable to regulatory government bodies. Radiofrequency regulations generally may not apply to the sensor 12 as thesensor 12 is a purely passive device.

The excitation pulse 14 does not require significant transmission timebecause a single short transmission of energy results in a single andcomplete sample of the ring signal 16. Power consumption may be reducedby using a lower transmission duty cycle, thereby reducing the dutycycle of transmit, receive, counting, and digital processing circuitry.By reducing power consumption battery power becomes a much more viableoption to power the system.

The excitation pulse 14 may be configured to maximize several systemparameters. For example, if a fixed frequency excitation pulse 14 isused, the frequency of the burst may be configured to maximizeparameters such as maximum allowable transmit peak power, maximumfreedom from in-band or near-band interference during the “receive”interval while the PLL is being locked to the ring signal 16, maximumworldwide acceptance of a particular frequency for reader transmissionsfor the desired sensor purpose, or other such criteria.

A level shifter 72 of the transmit circuitry 24 receives control signals54, 56, 58 and the RF signal 60 from the timing and control circuitry22. The level shifter 72 buffers the inputs and convert control logiclevels to circuit drive levels. A transmit driver 74 amplifies the RFsignal 60 to provide sufficient power to drive the antenna 26. The Qcontrol circuit 76 is activated during receive to reduce the Q of thecombined antenna 26 and tuning and D.C. block 82. A damping circuit 78is briefly activated immediately at the end of transmission of theexcitation pulse 14 to absorb energy in the antenna and allow theantenna to respond to the ring signal 16. The damping circuit 78 mayprovide a different Q factor to the antenna to improve reception of thering signal 16. The power control circuitry 80 controls the power-on andsleep mode for components in the transmit circuitry 24. The tuning andD.C. block 82 adjusts tuning for the antenna 26 and prevents directcurrent from improperly biasing the damping circuit 78. The RF output orexcitation pulse 14 from the transmit circuitry is routed to both theantenna 26 and the receive circuitry 28.

Once the excitation pulse 14 is transmitted by the transmit circuitry24, the receive circuitry 28 is configured to listen for the ring signal16. With reference to FIG. 5, a high Z buffer/clamp 84 includes a highimpedance (“high Z”) input device that limits the effect of the receivecircuitry 28 on the tuning performed by the tuning and D.C. block 82.The high Z buffer/clamp 84 further serves to protect the amplifierstages 86 from the extreme voltages present on the antenna 26 duringtransmission of the excitation pulse 14. Voltages at the antenna 26 mayreach upwards of 200 volts peak-to-peak during transmission of theexcitation pulse, requiring only approximately 60 pico-farads ofcapacitance to tune the antenna 26. In an embodiment, a 1 pico-faradcapacitor is used as a high impedance input current limiting device on a13.5 mega-hertz transmit circuit. Low capacitance diode junctions thatshunt over-voltage to the power supply and under-voltage to ground maybe used as clamping devices.

The amplifier stages 86 amplify the ring signal 16 to a sufficient levelto drive the PLL 30 input. Careful design of the amplifier stages 86 isrequired to achieve adequate transient response when the transmittedexcitation pulse 14 signal is removed and damped, and the low level ringsignal 16 is received. Common gate amplifier stages with low Q tunedreactive drain loads may be used to condition the high Z buffer/clamp 84output, followed by several filters interspersed between high gainamplifier stages. The filters may be either resistor-capacitor (“RC”)filters or inductor-capacitor (“LC”) filters. In an embodiment, thefilters may all be RC bandpass filters. Another common gate amplifierstage with low Q tuned reactive drain load may be used for finalbandpass conditioning prior to feeding the signal to the PLL 30 input.This design enables all of these amplifier types to perform fromextremely low signal input levels to extremely high signal input levelswithout signal distortion such as frequency doubling or halving due tostage saturation characteristics, as well as the excellent high inputimpedance achievable with the common-gate amplifier stages and theoutstanding transient response characteristics of the RC filterinterspersed between high gain amplifier stages. Special care must betaken in stage-to-stage power and signal isolation to prevent unwantedoscillations due to the extreme gain associated with the amplifierstages 86.

Power control circuitry 88 may apply and remove power to and from theamplifier stages 86 and the buffer in the high Z buffer/clamp 84 toreduce power consumption. It should be noted that the high Zbuffer/clamp 84 is designed to provide full protection even with powerremoved as excess energy will merely power up the amplifier stages 86until dissipated. The input impedance is high enough to limit excessenergy to prevent overpowering the amplifier stages 86. In anembodiment, the receive circuitry 28 is active during the transmissionof the excitation pulse 14 to decrease the time required for the PLL 30to lock onto the ring signal 16.

The PLL 30 receives the amplified and conditioned ring signal 16 fromthe receive circuitry 28. With reference to FIGS. 5 and 6, the RF signalfrom the receive circuitry 28 amplifier stages 86 feeds an RF buffer 90of the PLL 30. The RF buffer 90 may feed the RF signal to an optional RFdivider 92 that divides the RF signal frequency by an integer value.(FIG. 6.) The RF divider 92 then feeds the RF signal to a first input ofa phase frequency detector 94. The output of the frequency detector 94feeds a sample- and-hold (S/H) error amplifier 96. The S/H erroramplifier 96 controls the frequency of the VCO 32. The output of the VCO32 feeds the VCO divider 98, which output in turn feeds a second inputto the phase frequency detector 94. The PLL 30 may include an outputbuffer 102 to reduce loading of the VCO 32 while forwarding the VCOsignal frequency to the frequency counter 34. The VCO divider 98 allowsthe VCO 32 to operate at a frequency significantly higher than the ringfrequency 16. As a result, the time required to count and record the VCOsignal frequency may be significantly reduced. Moreover, the shortercount interval reduces VCO drift during counting and allows a highersample rate.

The phase frequency detector 94 is configured to determine the frequencyand phase error between the divided RF signal and the divided VCOsignal. This is best accomplished by filtering and amplifying the RFsignal that is fed to the S/H error amplifier 96. Further, the S/Hfeature may optimally forward the filtered and amplified signal tocontrol the VCO 32. In this manner, a closed control loop is formed thatcauses the VCO 32 frequency to equal to the ring signal 16 frequencytimes the VCO divider 98 integer divided by the RF divider 92 integer.The PLL 30 may include additional frequency dividers to optimize thecircuit design and increase the potential VCO 32 frequency range.

The PLL timer 48 sends a S/H mode control signal 66 to the S/H erroramplifier 96 of the PLL 30. The S/H mode control signal 66 may place theVCO 32 in a sample mode. In an embodiment, the VCO 32 is placed insample mode for a predetermined length of time. In sample mode, the VCOsignal frequency is adjusted to match the ring signal frequency, asdescribed above. When the S/H mode control signal 66 is placed in thehold mode, the S/H error amplifier 96 will hold its output constant,causing the control voltage to the VCO 32 to be approximately constantover a length of time sufficient to count the VCO signal frequency.

The power control signal from the PLL timer 48 to the power controlcircuitry 104 determines whether the PLL 30 is in a power on or asleep/power-off mode to conserve electrical power. Depending on thespecific PLL that is used, a control and communication link (not shown)may be required to set the RF divider 92 integer, the VCO divider 98integer, and the phase frequency detector 94 outputs and outputconfigurations. The communications link may be specific to theparticular PLL 30 used.

The frequency counter 34 includes counter stages 106, a counter buffer108, and a power control circuitry 110. The frequency counter timer 50sends a start/stop control input to the counter stages 106 and counterbuffer 108. The frequency counter timer 50 also sends a power controlinput to the power control circuitry 110. The counter stages 106 countthe VCO signal frequency from the PLL 30 output buffer 102. The counterstages 106 start counting when the start/stop control commands start,and end when the start/stop control commands stop. When the start/stopcontrol commands stop, the counter buffer 108 is loaded with the countvalue from the counter stages 106. The power control circuitry 110controls the power-on and sleep modes for components in the frequencycounter 34. The counter buffer 108 output may supply a count input tothe external interface circuitry 36. The ring frequency 16, andsubsequently the sensed parameter, may be determined from the frequencycount.

In operation, the reader 10 sequences as follows. During periods of timewhen the sensor 12 is not being sampled, all components of the reader 10are placed in reduced power mode. The wakeup timer 38 in the timing andcontrol circuitry 22 is configured for a particular sample delay orsample interval. At the specified time, the wakeup timer 38 initiates asample sequence. Specifically, the wakeup timer 38 powers up or wakes upeach component of the reader at appropriate times to ensure eachcomponent is in an operational state when needed.

The external interface circuitry 36 consumes minimal power when notactively communicating so is maintained in a ready condition at alltimes. The timing and control circuitry 22 provides the RF signal to thetransmit circuitry 24 for a short period of time, such as approximately20 microseconds. The RF signal from the timing and control circuitry 22is then secured and the transmit circuitry 24 is controlled to damp theantenna 26 quickly. The transmit circuitry 24 is then placed in anappropriate mode to allow reception of the ring signal 16 at the antenna26. In an embodiment, when the antenna 26 is configured to receive thering signal 16, the antenna 26 damping is greater than the ring signal16 damping.

During transmission of the excitation pulse 14, the receive circuitry 28receives, conditions, and clamps the transmitted RF signal at theantenna 26. Once transmission of the excitation pulse 14 ceases and theantenna 26 is configured to receive the ring signal 16, the receivecircuitry transitions into a high-gain reception mode to receive thering signal 16 from the antenna 26. The PLL 30 is in sample mode toallow the RF buffer 90 to receive the conditioned output of the receivecircuitry 28. When the antenna 26 begins to receive the ring signal 16,the PLL 30 shifts from locking onto the transmitted excitation pulse 14frequency to locking onto the ring signal 16 frequency. After a timeinterval sufficient for the PLL 30 to lock onto the ring signal 16frequency, the PLL 30 is shifted to hold mode to maintain VCO 32frequency at ring signal 16 frequency. The receive circuitry 28 andtransmit circuitry 24 are powered down or placed in sleep mode asappropriate.

Once the PLL 30 is in hold mode, the timing and control circuitry 22instructs the frequency counter 34 to conduct a controlled intervalcount of the VCO 32 frequency. Upon completion of the count, the PLL 30components are powered down or placed in sleep mode as appropriate andthe count value is transferred to the external interface circuitry 36.The frequency counter 34 components are then powered down or placed insleep mode as appropriate, and subsequently the timing and controlcircuitry 22 components are powered down or placed in sleep mode asappropriate. If programmed for interval sampling, the timing and controlcircuitry 22 wakeup timer 38 counts until the next sample is due.Otherwise, the timing and control circuitry 22 awaits a wakeup commandwith any other needed instructions from the external interface circuitry36.

The embodiment of the invention has been described above and, obviously,modifications and alternations will occur to others upon reading andunderstanding this specification. The claims as follows are intended toinclude all modifications and alterations insofar as they are within thescope of the claims or the equivalent thereof

1. A wireless sensor reader comprising: a transmit circuit configured togenerate an excitation pulse to cause a wireless sensor to emit a ringsignal; an antenna configured to transmit said excitation pulse andreceive said ring signal; a phase-locked loop circuit configured toreceive said ring signal, said phase-locked loop circuit including avoltage-controlled oscillator configured to generate a count signal at afrequency related to said ring signal frequency; wherein saidphase-locked loop circuit is capable of being placed in a sample mode toreceive said ring signal and adjust the frequency of said count signalbased on the frequency of said ring signal; and further wherein saidphase-locked loop circuit is capable of being placed in a hold mode tohold the frequency of said count signal constant for a length of timesufficient to determine the frequency of said count signal.
 2. Thewireless sensor reader of claim 1, wherein said excitation pulse is aradio frequency signal.
 3. The wireless sensor reader of claim 2,wherein said excitation pulse is a varied frequency pulse at or near theresonant frequency of said sensor.
 4. The wireless sensor reader ofclaim 2, wherein said excitation pulse is a fixed frequency pulse near afrequency that is harmonically related to the resonant frequency of saidsensor.
 5. The wireless sensor reader of claim 4, wherein saidphase-locked loop circuit further includes a frequency divider connectedto said voltage-controlled oscillator to allow said voltage-controlledoscillator to generate said count signal at a frequency higher than saidring signal frequency.
 6. The wireless sensor reader of claim 5, whereinsaid count signal frequency is an integer multiple of said ring signalfrequency.
 7. The wireless sensor reader of claim 6 further comprising areceive circuit connected to said antenna, said receive circuitconfigured to receive said ring signal from said antenna, amplify saidring signal, and send said ring signal to said phase-locked loopcircuit.
 8. The wireless sensor reader of claim 7 further comprising afrequency counter connected to said phase-locked loop circuitry, saidfrequency counter configured to determine the count signal frequency. 9.The wireless sensor reader of claim 8 further comprising a timing andcontrol circuit configured to place said transmit circuit, said receivecircuit, said phase-locked loop circuit, and said frequency counter intoa reduced power mode when not in use, and power up said transmitcircuit, said receive circuit, said phase-locked loop circuit, and saidfrequency counter at a predetermined time interval.
 10. The wirelesssensor reader of claim 9 further including a damping circuit configuredto absorb the energy in said antenna after transmission of saidexcitation pulse.
 11. The wireless sensor reader of claim 9 furthercomprising a Q control circuit to vary the bandwidth of said antennabetween a first bandwidth while said excitation pulse is transmitted anda second bandwidth while said ring signal is received.
 12. The wirelesssensor reader of claim 10 wherein said phase-locked loop circuit isconfigured to be in sample mode during transmission of said excitationpulse to allow said phase-locked loop circuit to lock onto the frequencyof said excitation pulse.
 13. A method of reading a remote sensorcomprising: transmitting an excitation pulse to a remote sensor;receiving a ring signal from said remote sensor in response to saidexcitation pulse; amplifying said ring signal; generating a countsignal; adjusting the frequency of said count signal to match thefrequency of said ring signal; and holding the frequency of said countsignal constant for a length of time to determine the frequency of saidcount signal.
 14. The method of reading a remote sensor of claim 13wherein said excitation pulse is an ultra wide-band RF signal pulse. 15.The method of reading a remote sensor of claim 14 wherein said remotesensor is a passive device.
 16. The method of reading a remote sensor ofclaim 15 wherein said ring frequency from said remote sensor varies inresponse to change in a specified parameter.
 17. The method of reading aremote sensor of claim 16 wherein said count signal is generated by avoltage-controlled oscillator.
 18. The method of reading a remote sensorof claim 17 wherein said count signal frequency is a numeric multiple ofsaid ring signal frequency.
 19. The method of reading a remote sensor ofclaim 18, wherein the control voltage of said VCO is placed into asample mode for a predetermined length of time to adjust the frequencyof said count signal to match the frequency of said ring signal, andafter said predetermined length of time, the control voltage of said VCOis placed into a hold mode to hold the frequency of said count signalconstant.
 20. The method of reading a remote sensor of claim 19 furthercomprising sending data related to the frequency of said count signal toan external data interface.